It is desirable for the substrate for a microelectronic device, integrated circuit, silicon chip to have the following properties for the indicated reasons:
1. High relative density in excess of 97.5% of theoretical maximum to provide a smooth surface. PA1 2. Low relative dielectric permittivity to allow rapid signal propagation rates. PA1 3. Low dielectric loss tangent to limit signal attenuation. PA1 4. High thermal conductivity to remove heat generated in the silicon chip PA1 5. Thermal expansion coefficient matched to that of the silicon chip to prevent breakage of the chip or of the adhesive joints or bonds between the chip and the substrate. PA1 6. A high modulus of rupture to prevent breakage of the substrate during handling such as with automatic assembly equipment or during insertion of contact pins into holes in the substrate. PA1 Step 1 --Silicon nitride powder, from about 2 to about 4 w/o stearic acid binder, from about 4 to about 7.5 w/o magnesium oxide, in methanol is ball milled for at least one hour with sintered silicon nitride milling media in polyethylene bottles to form a homogeneous slurry. PA1 Step 2 --The homogeneous slurry from step 1 is dried in an inert atmosphere at a temperature equal to or less than 60.degree. C. to form a dry powder. PA1 Step 3 --The dry powder from step 2 is ball milled with sintered silicon nitride media for a time sufficient to reduce the average aspect ratio of acicular crystals of the powder to less than 3. PA1 Step 4 --The product from step 3 is sieved through a 50 mesh sieve which is non-metallic. PA1 Step 5 --The product from step 4 is pelletized by pressing at a pressure sufficient to form a pellet. PA1 Step 6 --The product from step 5 is isostatically pressed from about 20,000 to about 26,000 psig. PA1 Step 7 --The product from step 6 is baked in air at 600.degree. C., for 15 hours to remove said stearic acid binder to form a green compact having a density from about 59% to about 65% of theoretical density. PA1 Step 8 --The product from step 7 is sintered at about 1750.degree. C. to about 1770.degree. C. in a nitrogen atmosphere having an overpressure sufficient to densify to greater than 98% of theoretical density to form a densified silicon nitride article. Alternatively, the product from step 7 is sintered at about 1650.degree. C. in a slowly flowing nitrogen atmosphere at ambient pressure to greater than 97.5% of theoretical density to form a densified silicon nitride article.
Sintered silicon nitride containing Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 and expansion coefficient similar to that of silicon has been used as a substrate for a silicon chip (J. Cummings, E. Anderson, H. Vora, and R. Wagner, Proceedings of VLSI Packaging Workshop, Gaithersburg, MD, 9/12-13/83). Silicon nitride containing MgO has been sintered without hot-pressing to a relative density of 97.5% (A. Giachello, P. C. Martinengo, G. Tommasini and P. Popper, J. Mat. Sci. 14(1979) 2825-2830) and, if formulated with 20w/o crystalline Si.sub.3 N.sub.4, to have a modulus of rupture in excess of 90,000 psi (U.S. Pat. No. 4,073,845). Hot pressed Si.sub.3 N.sub.4 /MgO was reported to have relative dielectric constant of 9.5 and dielectric loss tangent of 0.0055 at 0.1 MHz (J. S. Thorp and R. I. Sharif, J. Mat. Sci. 12(1977) 2274-2280). Hot-pressed Si.sub.3 N.sub.4 /MgO was reported to have thermal diffusivity of 0.08 cm s.sup.-1 at 500.degree. C. (T. G. Xi, Q. T. Chen, H. L. Ni, F. Y. Wu and T. S. Yen, Trans. J. Br. Ceram. Soc. 82 (1983) 175-177).
In addition, hot-pressed Si.sub.3 N.sub.4 /MgO was reported to have a thermal conductivity of 13.3 .times.10.sup.-3 cal cm.sup.-1 sec.sup.-1 K.sup.-1 or 55.6 Wm.sup.-1 K.sup.-1 (M. Mitomo, N. Hirosaki, and T. Mitsuhashi, Journal of Materials Science Letters 3 (1984) 915-916.